According to Plastics Industry:
There are a variety of methods used to process plastic. Each method has its advantages and disadvantages and are better suited for specific applications. These methods include: injection molding, blow molding, thermoforming, transfer molding, reaction injection molding, compression molding, and extrusion.
The main method used for processing plastic is injection molding. With this process, the plastic is placed into a hopper. The hopper then feeds the plastic into a heated injection unit, where it is pushed through a long chamber with a reciprocating screw. Here, it is softened to a fluid state.
A nozzle is located at the end of the chamber. The fluid plastic is forced through the nozzle into a cold, closed mold. The halves of the mold are held shut with a system of clamps. When the plastic is cooled and solidified, the halves open and the finished product is ejected from the press.
Thermosetting materials usually are not processed with injection molding because they will soften, they harden to an infusible state. If they are processed with injection molding, they need to be moved through the heating chamber quickly so they do not set.
Blow molding is used when the plastic item to be created needs to be hollow. A molten tube is created with blow molding by using compressed air, which blows up the tube and forces it to conform to the chilled mold. Variations of blow molding include injection, injection-stretch, and extrusion blow molding.
With injection blow molding uses a perform, which is taken to a blow mold and filled with compressed air. As a result, it conforms to the interior design of the blow mold. With injection-stretch blow molding, a the plastic is stretched prior to being formed. Otherwise, it is essentially the same as the injection process.
With continuous-extrusion, a molten plastic tube is continuously created. At the appropriate times, the tube is pinched between two mold halves. Then, a needle or a blow pin is inserted into the tube and blows compressed air up the part in order to force it to conform to the mold interior. With accumulator-extrusion, the molten plastic material is gathered in the chamber before it is forced through a die in order to form a tube.
Thermoforming uses a plastic sheet, which is formed with the mold by applying air or through mechanical assistance. The air pressure used can be nearly zero psi, or several hundred psi. At 14 psi, which is equivalent to atmospheric pressure, the pressure is created by evacuating the space between the mold and the sheet. This is known as vacuum forming.
Transfer molding is generally used only for forming thermosetting plastics. It is similar to compression molding because the plastic is cured into an infusible state through pressure and heat. Unlike compression molding, however, transfer molding involves heating the plastic to a point of plasticity prior to being placed into the mold. The mold is then forced closed with a hydraulically operated plunger.
Transfer molding was initially developed as a method for molding intricate products, such as those with many metal inserts or with small, deep holes. This is because compression molding sometimes disturbed the position of the metal inserts and the holes of these types of products. With transfer molding, on the other hand, the liquefied plastic easily flows around the metal parts without causing them to change position.
Reaction Injection Molding
Reaction injection molding, or RIM, is one of the newer processes used in the plastics industry. It differs from liquid casting in that the liquid components are mixed together in a chamber at a lower temperature of only about 75 to 140 degrees Fahrenheit before it is injected into a closed mold. Here, an exothermic reaction occurs. As a result, RIM requires less energy than other injection molding systems. Reinforced RIM, or R-RIM, involves adding materials such as milled or chopped glass fiber in the mixture in order to increase the stiffness.
Compression molding is the most common process used with thermosetting materials and is usually not used for thermoplastics. With this process, the material is squeezed into its desired shape with the help of pressure and heat. Plastic molding powder and other materials are added to the mix in order to create special qualities or to strengthen the final product. When the mold is closed and heated, the material goes through a chemical change that causes it to harden into its desired shape. The amount temperature, amount of pressure, and length of time utilized during the process depends on the desired outcome.
The process of extrusion is usually used to make products such as film, continuous sheeting, tubes, profile shapes, rods, coat wire, filaments, cords, and cables. As with injection molding, dry plastic material is placed into a hopper and fed into a long heating chamber. At the end of the chamber, however, the material is forced out of a small opening or a die in the shape of the desired finished product. As the plastic exits the die, it is placed on a conveyor belt where it is allowed to cool. Blowers are sometimes used to aid in this process, or the product may be immersed in water to help it cool.